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Algebraic and geometric theory of quadratic forms and symmetric bilinear forms, e.g., values attained by quadratic forms, isotropic subspaces, the Witt ring, invariants of quadratic forms, the discriminant and Clifford algebra of a quadratic form, Pfister forms, automorphisms of quadratic forms.

3 votes
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Diagonalization of quadratic forms over euclidean rings

Quadratic forms over $\mathbb{Z}$ don't diagonalize in general. Even positive definite rank two forms like $3x^2+2xy+5y^2$ can't be diagonalized. Inverting $2$ won't help things.
Robin Chapman's user avatar
8 votes
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Does a positive binary quadratic form represent a set of primes possessing a natural density

Accoring to H. Lenstra, Chebotarev's theorem holds both for Dirichlet and for natural density (but he doesn't give a reference in this document). Applying Chebotarev to the extension $H/\mathbb{Q}$ wh …
Robin Chapman's user avatar
4 votes
Accepted

Invariant quadratic forms of irreducible representations

There are certainly examples over $k=\mathbb{Q}$ where $\dim T\ge2$. Let's take the cyclic group $G$ of order $5$ and the representation space $$V=\{(a_0,\ldots,a_4)\in\mathbb{Q}^5:a_0+\cdots +a_4=0\} …
Robin Chapman's user avatar
9 votes

Root systems and sums of squares

If a quadratic form in $n$ variables is the sum of the squares of $n$ integer linear forms, it's the sum of the squares of $n$ rational linear forms. Thus it's equivalent as a rational quadratic form …
Robin Chapman's user avatar